This invention relates to downhole drilling systems for oil and gas exploration, and in particular, to a device known as an adjustable bent sub for altering the direction of a drill down a drill hole.
It is known in the prior art to provide subs for downhole drilling systems which are slightly bent in order to effect a change in vertical angle of direction of the drill bit. By providing a slight bend in the sub, the drill bit is encouraged to continue drilling at a slightly different angle below the sub than the angle made by the drill stem above the sub. In this manner, the drill bit may be made to follow a course which is not simply a vertical course but may be made to change direction to take account of changes in the composition of the earth, for example, to avoid certain obstacles or to drill out towards an underwater location.
It is desirable to provide an adjustable bent sub to avoid the necessity of having many different and expensive subs available on site, depending upon the angle of bend desired. It is much preferable to have a single unit which is capable of being adjusted to meet any required angle.
Trzeciak, U.S. Pat. No. 4,077,657 teaches one such adjustable bent sub. The sub is comprised of two longitudinal parts for passage of the drill stem therethrough, with mating faces between the parts, each such face being provided at an angle slightly off normal with respect to its respective axis. By rotating one of the parts about its axis relative to the other part, the degree of bend through the sub can be varied. Each mating face is provided with dog teeth and a lock nut is located internally to hold the two parts in a chosen relative angular displacement with respect to each other.
Wenzel, U.S. Pat. No. 4,745,982 also discloses an adjustable bent sub comprised of two parts with mating faces between the parts. However, unlike Trzeciak, the faces are normal to their axes, and a non-adjustable angular deviation is provided in the body of each part at a point somewhat removed from the mating faces in order to give the required deviation. In this case, the adjustable sub may be adjusted by disengaging the two parts, re-engaging the mating dog teeth of the two parts at the chosen relative angular displacement and re-securing the apparatus by means of lock nuts. However, when set to the 0.degree. position, the adjustable sub does not provide a straight through conduit for the drill bit but is always offset by a certain amount.
It is an object of the present invention to provide an adjustable bent sub apparatus which is simple to adjust accurately in the field to a desired angular deviation.